Voltage clamp

ABSTRACT

A voltage clamp circuit which operates using a voltage controlled current source where the change of the polarity of the voltage controlled current source controls whether it is clamping or not. While clamping, the stability of the control loop uses the capacitance of the output to create and single pole roll-off of the loop gain and while not clamping, uses the capacitance of the circuit which sets the clamping voltage to produce the roll-off. The circuit operates in a linear fashion both while clamping and not clamping, which allows for a faster response when clamping is needed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/082,841, titled VOLTAGE CLAMP and filed on Nov. 21,2014.

TECHNICAL FIELD

This disclosure relates generally to voltage clamps and, moreparticularly, to voltage clamps designed for dry circuit ohms clamping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a loop concept circuit on which certainvoltage clamp design embodiments in accordance with the disclosedtechnology are based.

FIG. 2 is a block diagram illustrating an example of a voltage clampcircuit coupled with a device under test (DUT) in accordance withcertain embodiments of the disclosed technology.

FIG. 3 is a block diagram illustrating an alternative example of avoltage clamp circuit coupled with a DUT in accordance with certainembodiments of the disclosed technology.

DETAILED DESCRIPTION

Dry circuit resistance generally requires a sourcing circuit that limitsthe maximum applied voltage across the device under test (DUT) to lessthan a few tens of millivolts (e.g., 20 mV to 30 mV). This is typicallyneeded to prevent the breaking down of a thin oxide that can form onelectrical contacts within the DUT, such as a relay contact or a contactwithin a connector pair.

Embodiments of the disclosed technology are generally directed to acircuit having a maximum voltage limit and a maximum current limit. Thecircuit may include a first current source configured to drive theoutput of the circuit (i.e., by setting the maximum current) and asecond current source having a polarity that is opposite the polarity ofthe first current source, and is coupled with a resistance.

A voltage-controlled current source may be configured to, when itscurrent has a polarity opposite that of the first current source, removecurrent from the output and, when its current has a polarity that isopposite that of the second current source, remove current from the nodecommon to the second current source and the resistance, thus reducingthe current through the resistance;

The circuit may include a component configured to compare the outputvoltage to the voltage across the resistance and drive the voltagecontrolled current source's input such than there is a negativefeedback.

The circuit may include a first capacitance added to the output of thecircuit to provide a single pole roll off of the output voltage from theinput to the voltage controlled current source. The circuit may alsoinclude a second capacitance added across the resistance to provide asingle pole roll off of the voltage across the resistance from the inputto the voltage controlled current source.

FIG. 1 illustrates an example of a loop concept circuit 100 on whichcertain voltage clamp design embodiments in accordance with thedisclosed technology are based. In the example 100, a capacitor C causesa 20 dB/dec roll-off where g_(m)/c represents the gain-bandwidth of thecircuit. The current source g_(m) should have a constant gain forfrequencies less than or equal to g_(m)/c.

FIG. 2 is a block diagram illustrating an example of a voltage clampcircuit 200 coupled with a DUT in accordance with certain embodiments ofthe disclosed technology. In the example, a current source I₀ is steeredby the two diodes D₁ and D₂ to regulate the voltage on one of twocapacitors C₁ or C₂. When the I₀ current is negative, the Ohm's currentsource I₁ is diverted through the first diode D₁, thus reducing thecurrent flowing to the HI connection.

When the I₀ current is positive, however, the Is current source issupplied through the second diode D₂, thus reducing the current flowingthrough a source resistor R_(s). The loop circuit 200 generally eitherregulates V₂/R₂ to be equal to −V_(z)/R_(z) (i.e., with the first diodeD₁ conducting) or regulates V_(z)/R_(z) to be equal to −V₂/R₂ (i.e.,with the second diode D₂ conducting).

In the example 200, a voltage V₁ may be clamped to R_(z)/R₂ R_(S)I_(S).Thus, R₂/R_(z) R_(S)I_(S) generally needs to be set between 20 mV and 30mV to meet the needs for dry circuit testing. The loop bandwidth isg_(m)/C₂ when D₁ is conducting and switches to g_(m)/C₂ when D₂ isconducting.

The ohm's current source protection will generally drop voltage whencurrent is flowing through it. Therefore, the capacitor C₂ and sourceresistor R_(s) may be bootstrapped to follow the ohm's current source'svoltage. This advantageously prevents both of the diodes D₁ and D₂ fromturning on at the same time. A differential stage, Diff, may be used totranslate the voltage across the source resistor R_(s) to ground to bemixed against the voltage V₁.

In the example 200, the first capacitor C₁ is used to limit the ratethat the voltage V₁ rises when a conduction path (e.g., the DUT) betweenthe HI and LO connections is suddenly removed, e.g., to minimize theovershoot. Using the capacitance—g_(m) interaction to control stabilitygenerally avoids problems that the capacitance may present in situationswhere a normal voltage source loop is used.

FIG. 3 is a block diagram illustrating an alternative example of avoltage clamp circuit 300 coupled with a DUT in accordance with certainembodiments of the disclosed technology. In the example 300, the clampvoltage for V₁ is I_(s)*R_(s).

Having described and illustrated the principles of the invention withreference to illustrated embodiments, it will be recognized that theillustrated embodiments may be modified in arrangement and detailwithout departing from such principles, and may be combined in anydesired manner. And although the foregoing discussion has focused onparticular embodiments, other configurations are contemplated.

In particular, even though expressions such as “according to anembodiment of the invention” or the like are used herein, these phrasesare meant to generally reference embodiment possibilities, and are notintended to limit the invention to particular embodiment configurations.As used herein, these terms may reference the same or differentembodiments that are combinable into other embodiments.

I claim:
 1. A voltage clamp circuit, comprising: a first current source;a second current source; a voltage-controlled current source; a firstdiode electrically coupled between the first current source and thevoltage-controlled current source; a second diode electrically coupledbetween the second current source and the voltage-controlled currentsource; a first protection device electrically coupled between the firstcurrent source and an input node configured to be electrically coupledwith a first port of a device under test (DUT); and a second protectiondevice electrically coupled between the second current source and theinput node, wherein the second protection circuit is also electricallycoupled between the voltage-controlled current source and the inputnode.
 2. The circuit of claim 1, further comprising a first resistanceelectrically coupled between the voltage-controlled current source andthe second protection device.
 3. The circuit of claim 2, furthercomprising a second resistance electrically coupled between the firstresistance and the second current source, wherein the second resistanceis also electrically coupled between the voltage-controlled currentsource and the second current source.
 4. The circuit of claim 3, furthercomprising a third resistance electrically coupled between the firstcurrent source and the second current source.
 5. The circuit of claim 4,further comprising a first capacitance electrically coupled between thefirst current source and the second current source.
 6. The circuit ofclaim 5, further comprising a second capacitance electrically coupledbetween the first protection device and ground, wherein the secondcapacitance is also electrically coupled between the second protectiondevice and ground.
 7. The circuit of claim 6, further comprising adifferencing device, the differencing device including: a first inputelectrically coupled with the first current source; a second inputelectrically coupled with the second current source; and an outputelectrically coupled with the second resistance.
 8. The circuit of claim7, further comprising a ground node configured to be electricallycoupled with a second port of the DUT.
 9. The circuit of claim 8,wherein the second capacitance is also electrically coupled between theinput node and the ground node.
 10. The circuit of claim 9, furthercomprising a first amplifier electrically coupled between the firstcurrent source and the differencing device.
 11. The circuit of claim 10,further comprising a second amplifier electrically coupled between thesecond protection device and the first resistance.
 12. A voltage clampcircuit, comprising: a first output node configured to be electricallycoupled with a first port of a device under test (DUT); a second outputnode configured to be electrically coupled with a second port of theDUT; a first current source connected to the first output node; a secondcurrent source connected to the second output node, the second currentsource having a polarity that is opposite a polarity of the firstcurrent source; a voltage-controlled current source; circuitryconfigured to couple the voltage-controlled current source to the firstcurrent source when a polarity of the voltage-controlled current sourceis opposite the polarity of the first current source; circuitryconfigured to couple the voltage-controlled current source to the secondcurrent source when the polarity of the voltage-controlled currentsource at is opposite the polarity of the second current source; aresistor connected to the second current source and configured toconduct current from the second current source and current from thevoltage-controlled current source when the polarity of thevoltage-controlled current source is opposite the polarity of the secondcurrent source; and circuitry configured to drive the voltage input ofthe voltage controlled current source with a difference of the voltageacross the resistor and the voltage across the first and second outputnodes.
 13. The voltage clamp circuit of claim 12, further comprising: afirst protection device electrically coupled between the first currentsource and the first output node; and a second protection deviceelectrically coupled between the second current source and the firstoutput node, wherein the second protection circuit is also electricallycoupled between the voltage-controlled current source and the firstoutput node.